Non destructive editing.
You can add a mask to a layer and use the mask to hide portions of the layer and reveal the layers below. Masking layers is a valuable compositing technique for combining multiple photos into a single image or for removing a person or object from a photo.
1. Open the channels of your image selecting the image with the most contrast in it – most probably the blue. Duplicate it, only work on this image.
2. Go to adjustments, select the levels, and push the image so it goes darker. Be careful not to go black.
3. Then experiment with the burn tool to make the image darker, finishing off with the dodge tool to whiten the edges of the image.
4. Once your mask is clearly black and white, select command, and returning to the layers of your image, add a mask (which is located at the bottom of the layers palette).
5. You should now have a layer mask, to test it, create another layer and fill it with a solid colour) ensure it is behind the image you have been working on.
6. For finer adjustments, double click the mask you have just made, experiment with the ‘mask edge’, especially if your image has hair or a rough edge.
An adjustment layer applies colour and tonal adjustments to your image without permanently changing pixel values. For example, rather than making a Levels or Curves adjustment directly to your image, you can create a Levels or Curves adjustment layer.